NL8301979A - ZINC SUBSTANCE BY WET SPRAY AND DISTRIBUTION. - Google Patents

Description

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Zinc dust by wet spraying and distribution.

The invention relates to the spraying of zinc into zinc dust and its distribution for use in zinc mill operations, for example in the purification of zinc sulfate solutions and the formation of cadmint-5 sponge.

In zinc plant operations, zinc concentrate is normally roasted and the calcined product is then leached with sulfuric acid. The zirconium sulfate solution thus formed is first purified by oxidation and hydrolysis to eliminate iron and other hydrolyzable impurities and is further treated to eliminate impurities such as copper, cadmium and cobalt before the solutions are electrolysed to recover zinc. The purification of the zinc sulfate solution to remove copper, cadmium and cobalt ions generally takes place by depositing on (binding to) zinc dust. A detailed description of such a purification process is found in U.S. Patent 4,168,970.

The yield of the deposition or bonding process depends largely on the activity of the zinc dust used for deposition or bonding, which activity in turn is a function of the morphology and composition of the zinc dust. Today, zinc dust is prepared by an air spray technique.

However, the efficiency of the air atomizing technique for the production of fine zinc dust is limited and, as a result, large amounts of zinc dust are required to achieve acceptable impurity removal. In addition, the use of air-sprayed zinc dust 3Q in the purification poses a dust problem and explosion hazard.

The object of the invention is therefore to provide an improved process and an improved system for the preparation and distribution of zinc dust for use in operations in a zinc factory.

Surprisingly, it has now been found that zinc dust formed by wet road, prepared by spraying with water or other suitable liquid medium under pressure, exhibits greater activity than zinc dust obtained by conventional air spraying techniques. It is believed that the greater efficiency of zinc dust sprayed by liquid is due to its smaller average particle size as compared to zinc dust obtained by air spraying, which is reflected in a larger specific surface area per unit weight. In addition, liquid dusting zinc dust is more active than air dusting zinc dust, because the particles have a greater specific surface area than the air dusting particles, due to their very irregular shape.

The method of preparing and distributing wet-formed zinc dust over a zinc sulfate purification system or other operations carried out in a zinc plant, because the atomization of molten zinc using water or other suitable liquid medium under high pressure and supplying a certain amount of the zinc dust formed in the wet way to at least one tank 25 for the purification of zinc sulphate or to another operation in a zinc factory. If it is desired to remove the excess liquid medium in order to maintain the factory water balance, the atomizer zinc dust is introduced into a liquid / solid separator to remove the excess liquid and the thus formed heavy suspension (slurry) resuspended with a suitable solution to form a dilute zinc dust suspension containing a certain percentage of zinc dust.

The resuspending solution may consist of a purified, a partially purified or even an impure zinc sulfate solution, depending on the processing in the zinc plant involved.

The molten zinc can contain small amounts of other alloying elements, such as lead, cadmium, aluminum, copper, iron, etc.

A preferred wet dusting and preparation system for zinc dust and its distribution includes means for spraying molten zinc or a suitable zinc alloy with water or another suitable liquid medium under high pressure and means for supplying predetermined amount of the zinc dust prepared by wet route to at least one zinc sulfate purification tank or other processing operation at a zinc plant.

If it is desired to remove the excess liquid medium, in order to maintain the factory water balance, means may be available for removing excess liquid from the zinc dust thus prepared and for resuspending the heavy suspension which is thereby formed to obtain a dilute suspension containing a predetermined concentration of zinc dust particles.

The zinc dust slurry is preferably circulated continuously and (predetermined) amounts thereof are introduced into the purification tank or tanks or other operations of the zinc plant.

The invention is now further described and elucidated by way of example with reference to the drawings, in which:

Figure 1 shows a block diagram of a water spraying system for preparing water-sprayed zinc dust and distributing (supplying) water-sprayed zinc dust over purification tanks;

Figure 2 shows a schematic of a purification pilot plant as used to conduct tests under conditions that closely mimic the operating conditions used at Canadian Electrolytic Zinc, Quebec, Canada; and 8301979

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4

Figure 3 shows a graph showing the consumption of the atomization with air or the atomization with water-formed zinc dust as a function of the concentration of Co impurities in an impure zinc sulfate solution.

5 Trials conducted with a D-75 water-based atomizer from Davy-Loewy Ltd have shown that it can reliably produce fine zinc dust using high pressure water between 10.5 MPa and 21 MPa at a flow rate of molten zinc in the range of 40-60 kg / min.

10 and with a water / metal weight ratio greater than 2: 1,

Water pressures of about 14 MPa and a water / metal weight ratio in the range of 3-4: 1 are preferred.

A comparison of the zinc dust particles formed by spraying with water as described above with the air sprayed zinc dust particles normally used in Canadian Electrolytic Zinc, Quebec, Canada has shown that the zinc dust particles formed by spraying with water (80-90% passes through a 0.147 mm mesh sieve) than the air-atomized zinc dust particles (65-80% passes through a 0.147 mm mesh sieve), which means zinc dust particles with a larger specific surface area per unit weight .

It was also found that the zinc dust particles formed by spraying have a larger specific surface area due to their very irregular surfaces. For example, zinc dust sprayed with water (90% having a particle size of less than 0.147 mm) had twice the specific surface area of an air dusted zinc dust (85% of which had a particle size of less than 0.147 mm). Of course, other water-based atomizers suitable for the preparation of zinc dust of comparable quality can also be used.

Figure 1 shows schematically a block diagram of a preferred system for atomizing using water and for distributing.

This system comprises at least one unit 10, for dusting 8301979 f% 5 using water suitable for the production of zinc dust with a particle size such that at least 70% and preferably at least 90% passes through a sieve with a mesh size of 0.147 mm, at a rate of at least 3.5 tonnes per hour in continuous operation. A flow of liquid zinc at a temperature of 450-500 ° C is atomized in the atomizing unit using high pressure water jets and at a water to metal weight ratio in the preferred range of 3-4: 1. The supply of liquid zinc to the atomizer may consist of a zinc alloy containing lead, cadmium, aluminum, copper, iron, etc. For the purification of zinc sulfate, the liquid zinc feed preferably consists of a zinc alloy containing 0.5 to 2% by weight of lead. To minimize their impact on the environment, modern zinc electroplating plants have been designed to operate with a very precisely hand-held water balance, producing a minimum of waste liquids.

To control the factory water balance, the zinc-20 dust formed in the atomizer 10 by spraying with water is supplied by pump or gravity to a separation stage 12 for separating liquid and solid , which may include classifiers, compactors, clarifiers, thickeners, decanters, or filters to remove excess water from the zinc dust formed by water spraying. The liquid-solid separation step is preferably carried out with a classifier / densifier equipped with means for recovering the settled zinc dust as a heavy slurry containing about 20% water. The excess water from the liquid / solid separation stage can be used for washing operations in other parts of the plant or can be disposed of as wastewater. Zinc dust sludge (heavy zirconium slurry) is withdrawn from the liquid / solid separation stage and fed to a controlled resuspending slurry stage 14, which resuspending stage preferably comprises a batch operating tank 8301979 6 in which a zinc density slurry of a certain density is prepared by mixing with a resuspending solution, for example, a purified, a partially purified or even an impure zinc sulfate solution, depending on the operation at the zinc plant. The resuspended zinc dust slurry is then fed to the purification tank or tanks, which is indicated in the block diagram as "divide over purification tank (s)". Preferably, the slurry is continuously recirculated through a manifold system from which a metered amount is fed to the appropriate factory site. It will be clear that other equivalent schemes for the preparation of zinc dust by spraying and for the distribution (distribution) of zinc dust can also be used and that the scheme discussed above is no more than an example of a possible mode of operation. . For example, if the water balance is not critical, the water-sprayed zinc dust obtained in the atomizer 10 can, for example, be fed directly to the distribution stage 16. The water-dusted zinc dust can further be used in any zinc plant operation applied where zinc dust is used to deposit or bind impurities on it.

Factory-scale purification tests were conducted under continuous treatment conditions that closely correspond to the factory operating conditions at Canadian Electrolytic Zinc, Quebec, Canada, to evaluate the efficiency (achieved efficiency) of the water-atomized zinc dust. tests were also conducted with zinc dust prepared by air spraying as used in Canadian Electrolytic Zinc, Quebec, Canada to allow a comparison between the efficiency of both types of zinc dust.

3

A crude starting solution (about 50m 3 from the factory) was stored in a 150m 35 tank and used in the two series of purification tests. This mode of operation made it possible to use the same quality crude starting solution for both series of tests Ce.

The average composition of the crude starting solution used in these experiments was 150 g / 1 Zn, 0.54 g / 1 Cu, 24 mg / 1 Co and 0.77 g / 1 Cd.

5 The purification system included three purification stages with a total of seven tanks (each tank with a volume of 80 liters) as shown in figure 2. The crude starting solution was pumped through the system by means of a pump 20 connected to a storage tank 22 , with a flow rate of 2 1 / min. which led to a residence time of 40 min. per tank. The first stage of the purification was simulated using two cascaded tanks 24 and 26 in which the copper and cadmium impurities, along with about 70% of the cobalt, were deposited on (bound to) the recycled purification residue ( 90% unconverted zinc) from the second and third stages. After the first stage purification, the solution was passed to a thickener 28 to separate a solid residue containing the copper, cadmium and about 70% of the cobalt. About 50% of this residue was subjected to further treatment to recover copper and cadmium and the remainder was returned to the first tank 24. The overflow solution from the thickener 28 was then treated in a second purification stage containing four tanks 30, 32, 34 and 36 which were cascaded and served to remove the cobalt-25 contamination. This purification was performed by depositing on (binding to) zinc dust which was fed to the first tank 30 (about 70%) and to the third tank 34 (about 30%). Antimony trioxide (1.7 mg / l) was added to the first tank. The solution containing the purification residue was pumped to a thickener 38 with a pump 40 to separate the solid there.

The thickener overflow was finally treated in a third stage which included purification tank 41 and served to remove any traces of impurities. This purification step was performed by adding zinc dust. The solution in tank 4J overflowed to a storage tank 42 and was then pumped to a filter press 44 by pump 46 and the filtrate 8301979 8 was analyzed for the presence of the various elements, i.e. copper, cadmium, cobalt. The underflow of the second stage thickener 38 together with the solid from the filter press 44 was recycled to the first stage 24 first tank. The tanks were kept at a desired temperature by electric heaters 48. The tanks were also equipped with stirrers 49 to stir the solution into the tanks. A detailed representation of the operating conditions prevailing during these 10 tests is shown in Table A below. The particle size of the two types of zinc dust used in these tests was: 76.3% passed through a 0.147 mm mesh screen in the case of the zinc dust atomized by air and 86.4% passed through a 0.147 mm mesh screen in the case of zinc dust formed by water atomization.

20 8301979 9

T ^ ball A

Experimental Zinc dust_ conditions with air atomized with water atomized (1% Pb) steamed (1¾ Pb)

Impure solution flow rate (I / min) 2.0 2.0 density 1.380 1.380 Co ++ mg / 25 25 25

Residence time -le stage (min) 80 80 -thicker no.1 (min) 100 100 -2nd stage (min) 160 160 -thicker no.2 (min) 100 100 -3rd stage (min) 40 40 -Total (h) 8 8

Temperature

-le stage 70 + 2 ° C 70 + 2 ° C

-2nd stage 98 + 2 ° C 98 + 2 ° C

-3rd stage 98 + 2 ° C 98 + 2 ° C

25_ -2nd stage 3.85 + 0.15 no comparison first reservoir (4.2) -3® stage 3.85 + 0.15 3.85 + 0.15

St2 ° 3. , e -xn 1 second-stage reservoxr (mg / 1) 2 2

Solid recirculation

a

-thicker I stage 50% to the first- 50% to the first stage 1st stage -thicker 2nd stage 600 ml / min to 600 ml / min to the first stage the first stage filter press 4 1 / h to 4 1 / h to the first stage first stage.

Typical results of the pilot plant scale purification tests using different amounts of zinc dust are listed in Table B for tests with zinc dust obtained by air spraying and in Table C for tests performed with zinc dust obtained by water spraying.

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Analysis of these figures, based on the frequency of tests with a residual concentration of less than 0.2 mg / l, showed that the use of zinc dust by spraying with water is required equivalent to 9.5% of the production of cathodic zinc, to achieve the same degree of purification as obtained with the use (consumption) of air atomized zinc dust in an amount equivalent to 14.3% of the cathodic zinc production. This means a saving of about 33% in Zirk-dust consumption. It was also found that this zinc dust savings are valid for a range of cobalt concentrations in the crude starting solution as shown in Figure 3 due to the practically constant distance between the two curves. These results were obtained in separate experiments in a pilot plant using crude zinc sulfate solutions containing about 10 mg / l, 14 mg / l, 20 mg / l and 24 mg / l cobalt in solution.

20 8301979

Claims (10)

12 1. Process for the production of zinc dust for use in zinc plant operations, characterized in that molten zinc is atomized using water or other suitable liquid medium under high pressure. 2. Process according to claim 1, characterized in that the particles of the zinc dust prepared in the wet road have a large specific surface area and such a particle size that at least 70% passes through a sieve with a mesh size of 0.147 mm. 3. A method according to claim 1 or 2, characterized in that the zinc dust prepared in the wet way is used in operations in a zinc factory in which a certain amount of the zinc dust formed in the wet way is supplied to at least one processing step of the operations taking place in the zinc factory. Method according to claim 3, characterized in that the water balance is controlled in the factory by removing excess liquid from the zinc dust formed by spraying to form a heavy suspension and by adding a resuspending solution to the heavy suspension and thus prepare a dilute suspension with a predetermined concentration of zinc dust, which is used to feed the desired operation. Process according to claim 4, characterized in that the resuspending solution is a purified, partially purified or crude zinc sulfate solution. 6. Process according to claims 1-5, characterized in that the molten zinc is a zinc alloy which also contains other metals such as lead, copper, cadmium, aluminum and iron. 7. System for wet spraying of atomized zinc dust and for distribution (feeding to certain stages), comprising means for spraying molten zinc or a suitable zinc alloy using 8301979 I * V. * water or another suitable high pressure liquid medium and means for distributing (supplying) a given amount of the wet-form silica to at least one zinc plant operation. System for the production of zinc dust by spraying and distributing it according to claim 7, characterized in that means are present for removing excess liquid from the zinc dust thus prepared to form a heavy suspension and medium - 10 parts are present for adding a resuspending solution to the heavy suspension to prepare a dilute suspension containing a certain concentration of zinc dust controlling the factory water balance. 9. A system for preparing and distributing zinc dust by spraying with liquid according to claim 8, characterized in that the means for removing excess liquid from the zinc dust consist of a classifying / compacting device equipped with means for recovering of the settled dust as a heavy suspension 20 containing about 20% water. 10. System for spraying and distributing zinc dust by liquid spraying according to claim 8 or 9, characterized in that means are present for circulating the suspension and for draining from the circulation system a certain amount of the slurry to an operation in the zinc plant. 30, 8301979